The long term goal of our research is to develop an acoustically-based, explanatory model of the communication deficit in dysarthria that can be used to guide and justify treatment decisions. Toward this end, the proposed Phase I treatment project will investigate the relationship among phonatory and supralaryngeal acoustic measures of speech, intelligibility, and speaking conditions used as intervention strategies for dysarthria secondary to Parkinson disease and Multiple Sclerosis. Studies from the first funding cycle indicated that vowel distinctiveness was maximized in a Slow condition while consonant distinctiveness and intelligibility were maximized in a Loud condition. Supralaryngeal acoustic measures also accounted for only a portion of the variance in intelligibility. Whether a speech mode encouraging a slowed rate and increased intensity would yield improvements in acoustic-phonetic distinctiveness and intelligibility above those associated with rate reduction or increased loudness alone is unknown, although the Perceptual-Acoustic Theory (Perkell et al., 2000) predicts such an outcome. The proposed project tests this and other predictions of the Perceptual-Acoustic Theory by extending the study of speech mode effects in dysarthria to Clear speech, a speech mode encouraging a slowed rate and increased intensity. The contribution of acoustic measures of phonatory behavior to intelligibility as well as measures of acoustic-phonetic distinctiveness also will be studied. Loud, Slow, Clear, and even Fast speech modes are used therapeutically to maximize intelligibility in dysarthria, yet comparative group studies are lacking. Research that improves our understanding of acoustic-perceptual changes associated with these speech modes would strengthen the scientific bases of treatment techniques and may reveal acoustic-perceptual advantages of a given speech mode that will determine preferred therapies - key considerations for evidence based practice. The overarching hypothesis to be evaluated is that intelligibility and acoustic-phonetic distinctiveness will be maximized in conditions associated with increased effort, as indexed by SPL, and for which a slowed rate is encouraged, whereas phonatory function will be maximized in conditions associated with increased effort. This hypothesis is suggested by the Perceptual-Acoustic Theory, which posits a trade-off between effort and acoustic-perceptual adequacy as well as articulatory rate and accuracy.